TLS(2) BSD System Calls Manual TLS(2)NAME
set_tls_area, get_tls_area — kernel TLS (thread local storage) support
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/tls.h>
int
set_tls_area(int which, struct tls_info *info, size_t infosize);
int
get_tls_area(int which, struct tls_info *info, size_t infosize);
DESCRIPTION
The set_tls_area() system call creates an entry for the TLS facility
which representing thread local storage as specified by the info struc‐
ture. A descriptor representing the facility is returned, or -1 if an
error occurred. The facility may be cleared by specifying a NULL pointer
and an infosize of 0. The get_tls_area() system call retrieves the
requested TLS facility. A descriptor representing the facility is
returned, or -1 if an error occurred. If you simply want the descriptor
you may specify a NULL pointer and an infosize of 0.
The returned descriptor and the TLS mechanism is machine-dependent. On
IA32 three global segment descriptors are supported (0, 1, and 2) and
the %gs load value is returned.
The tls_info structure passed to set_tls_area() should first be zerod (to
remain compatible with future extensions) and then initialized.
struct tls_info {
void *base; /* base address of TLS area */
int size; /* size of TLS area in bytes */
};
The actual implementation of the area is machine-dependent. If the ker‐
nel is unable to accommodate the supplied size it may create a larger
area. If the kernel is unable to accommodate the supplied base address
an error will be returned.
RETURN VALUES
A return value of 0 is returned on success, -1 on error.
EXAMPLE
/*
* Pseudo example showing how the TLS system calls work on IA32.
*/
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/tls.h>
int X;
static int getdata(int offset);
int
main(int ac, char **av)
{
int i;
int gs;
struct tls_info info;
info.base = &X;
info.size = sizeof(X);
if ((gs = set_tls_area(0, &info, sizeof(info))) < 0) {
perror("setarea");
exit(1);
}
printf("gs = %04x\n", gs);
__asm __volatile("mov %0,%%gs" : : "g" (gs) );
if (get_tls_area(0, &info, sizeof(info)) < 0) {
perror("getarea");
exit(1);
}
printf("%p/%d\n", info.base, info.size);
X = 1;
printf("should be 1: %d\n", getdata(0));
X = 2;
printf("should be 2: %d\n", getdata(0));
printf("this should fault:\n");
fflush(stdout);
getdata(4);
return(0);
}
static int
getdata(int offset)
{
int rv;
__asm __volatile("movl %%gs:(%0),%%eax; movl %%eax,%1" : "+r" (offset) : "m"
(rv) : "ax");
return (rv);
}
ERRORS
[ERANGE] The specified facility index, which, is not supported.
[EINVAL] An invalid parameter has been specified.
[ENOENT] (get_tls_area) The specified facility has not been
initialized with sys_set_tls_area().
SEE ALSOumtx(2)HISTORY
The set_tls_area(), and get_tls_area() function calls first appeared in
DragonFly 1.1.
BSD February 21, 2005 BSD
